The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook , Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
Electronic packages extend from the integrated circuit chip, through the module, card, and board, to the gate and system. The integrated circuit "chip" is referred to as the "zero order package". This chip or zero order package enclosed within its module is referred to as the first level of packaging. The integrated circuit chip provides circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical integrity.
There is at least one further level of packaging. The second level of packaging in the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip, and consequently, multiple chips are required to perform the function. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation. Several cards may, in turn, be mounted on one board.
Cards and boards may be polymer based or ceramic based. A basic process for polymer based composite package fabrication is the "prepreg" process described by George P. Schmitt, Bernd K. Appelt and Jeffrey T. Gotro, "Polymers and Polymer Based Composites for Electronic Applications" in Seraphim, Lasky and Li, Principles of Electronic Packaging, pages 334-371, previously incorporated herein by reference, and by Donald P. Seraphim, Donald E. Barr, William T. Chen, George P. Schmitt, and Rao R. Tummala, "Printed Circuit Board Packaging" in Tummala and Rymaszewski, Microelectronics Packaging Handbook, pages 853-922, also previously incorporated herein by reference.
In the "prepregging" or impregnation process for polymeric electronic circuit package fabrication, a fibrous body, such as a non-woven mat or woven web, is impregnated with a laminating resin. This step includes coating the fibrous body with, for example, an epoxy resin solution, evaporating the solvents associated with the resin, and partially curing the resin. The partially cured resin is called a B-stage resin. The body of fibrous material and B-stage resin is the prepreg. The prepreg, which is easily handled and stable, may be cut into sheets for subsequent processing.
Typical resins used to form the prepreg include epoxy resins, cyanate ester resins, epoxy cyanate blends, bismaleimides, polyimides, hydrocarbon based resins, and fluoropolymers. The epoxide compounds used to prepare prepregs comprise in general those compounds containing at least one vicinal epoxy group, i.e., at least one ##STR2## group. Examples of suitable epoxy resins for prepregs are represented by the general formula: ##STR3##
The epoxies may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic and contain if desired substituents such as halogen atoms, hydroxyl groups, ether radicals and the like. These systems may be monomeric, oligomeric or polymeric in nature. Suitable epoxides are disclosed in the U.S. Pat. Nos. 3,356,624; 3,408,219; 3,446,762 and 3,637,618.
Preferred epoxides are the glycidyl polyethers of polyhydric phenols and polyhydric alcohols, especially the glycidyl polyethers of 2,2-bis(4-hydroxyphenyl) propane having an average molecular weight between about 300 and 3,000 and an epoxide equivalent weight between about 140 and 2,000 and more preferably an epoxide equivalent weight of from about 140 to about 650.
Preferably the base resin which has an epoxide equivalent weight of from 175 to 195, is derived from condensing epichlorohydrin with 2,2-bis(4-hydroxyphenyl)propane to form 2,2-bis((p-2,3 epoxypropoxy) phenyl) propane.
A widely used class of epoxides includes the epoxy polyethers obtained by reacting an epihalohydrin, such as epichlorohydrin with either a polyhydric phenol or a polyhydric alcohol. Some examples of dihydric phenols include 4,4'-isopropylidene bisphenol, 2,4'-dihydroxydiphenylethyl methane, 3,3-dihydroxydiphenyldiethylmethane, 3,4'-dihydroxydiphenylmethylpropyl methane, 2,3'-dihydroxydiphenylethylphenylmethane, 4,4'-dihydroxydiphenylpropylphenylmethane, 4,4'-dihydroxydiphenylbutylphenylmethane, and the like.
Another class of polymeric epoxides represented by the general formula shown below, includes the epoxy novolac resins obtained by reacting, preferably in the presence of a basic catalyst, e.g. sodium or potassium hydroxide, an epihalohydrin, such as epichlorohydrin, with the reaction product of an aldehyde, e.g. formaldehyde, and either a monohydric phenol or a polyhydric phenol. Further details concerning the types and preparation of epoxy resins can be obtained in Lee, H. and Neville, K., Handbook of Epoxy Resins, McGraw Hill Book Co., New York, 1967. Typical formulae are: ##STR4## where each X' is independently a divalent hydrocarbon group having from 1 to about 6, preferably from 1 to about 4 carbon atoms, each R' is independently hydrogen or an alkyl group having from 1 to about 4 carbon atoms; n' has a value of from about zero to about 30, preferably from zero to about 5; n" has a value of from about 0.001 to about 6, preferably from about 0.01 to about 3; x' has a value of 3, each R is independently hydrogen a hydrocarbyl or hydrocarbyloxy group having from 1 to about 10, preferably from 1 to about 4 carbon atoms, halogen, preferably chlorine or bromime.
Particularly suitable epoxides which can be employed herein include, for example, the diglycidyl ethers of resorcinol, bisphenol A, 3,3',5,5'-tetrabromobisphenol A, the triglycidyl ether of tris(hydroxylphenyl) methane, the polyglycidyl ether of a phenol formaldehyde condensation product (novolac), the polyglycidyl ether of a dicyclopentadiene and phenol condensation product and the like.
One type of prepreg is the FR-4 prepreg. FR-4 is a fire retardant epoxy-glass cloth material, where the epoxy resin is the diglycidyl ether of 2,2'-bis(4-hydroxyphenyl) propane. This epoxy resin is also known as the diglycidyl ether of bisphenol-A, (DGEBA). The fire retardancy of the FR-4 prepreg is obtained by including approximately 15-20 weight percent bromine in the resin. This is achieved by incorporating the appropriate amount and type of resins or other brominated compounds.
Still other bisphenol A diglycidyl ether resins are used to form rigid circuit boards. Among the resins so used to produce "prepreg" for reinforced laminate compositions for circuit boards are the lower molecular weight bisphenol A diglycidyl ether resins, including bromine-substituted resins for imparting some degree of flame resistance as disclosed by U.S. Pat. No. 4,782,116. Such epoxy resins are of relatively low equivalent weight, in the area of 180 to 200, using non brominated resin for example, so that the epoxy group content is relatively high, i.e., each relatively short repeating unit contains two epoxy groups, which results in an increase in the dielectric constant of the compositions after curing.
Other epoxy resin formulations useful in providing prepregs include high functionality resins, such as epoxidized cresol novolacs, and epoxidized derivatives of tris-(hydroxyphenyl) methane. The multifunctional epoxy resins are characterized by high glass transition temperatures, high thermal stability, and reduced moisture up take.
Phenolic cured epoxies such as Ciba-Giegy RDX86-170, Ciba-Giegy RD787-211, Ciba-Giegy RD87-212, Dow Quatrex 5010, Shell Epon 1151, and the like are also used in forming prepregs. These products are mixtures of epoxies, with each epoxy having a functionality of at least 2, and an imidazole catalyst.
Cyanate resins are also used in forming prepregs. One type of cyanate ester resin includes dicyanates reacted with methylene dianiline bis-maleimide. This product may also be reacted with compatible epoxides to yield a three component laminate material. One such laminate material is a 50:45:5 (parts by weight) of epoxy: cyanate: maleimide. Typical of cyanate ester resins useful in forming prepregs is the product of bisphenol A dicyanate and epoxy, which polymerizes during lamination to form a crosslinked structure.
Another class of dielectric substrates are film adhesive systems. These differ from above described fiber-resin systems by the use of an adhesive bearing film. One polyimide used for the film to carry the adhesive in a film adhesive system, is a polyimide based on diphenylene dianhydride, described in U.S. Pat. No. 4,725,484 to Kiyoshi Kumagawa, Kenji Kuniyasu, Toshiyuki Nishino, and Yuji Matsui for Dimensionally Stable Polyimide Film and Process for Preparation Thereof. This patent describes a copolymer of 3,3',4,4'-biphenylenetetracarboxylic dianhydride and p-phenylene diamine, commercially known as Upilex S.
Some proposed adhesive mixtures contain substantial amounts of the epoxy resin relative to the dicyanate polymer(s), producing an even higher dielectric constant. Also in such compositions the glass transition temperature and processing or curing temperature generally are reduced to such an extent that the thermal stability of the cured prepregs or laminates is unsatisfactory for high temperature processing applications.
However, the presently known film adhesive systems and fiber adhesive systems suffer from shortcomings. For example, epoxy-glass systems have a relatively high dielectric constant, and relatively poor thermal stability, while polyimide glass systems have a poor copper peel strength. Attempts to substitute polymeric fibers or films for the glass fibers have introduced problems of microcracking and poor mechanical properties.